I'm upgrade from a pont & shot and I'm a little disappointed with J1... I don't think the quality of the images far superior from P&S and even from camera phone such an Iphone or GS3.

I own J1 with 10-30mm kit lens. i wanna take pictures with background defocus and think it's not possible. It's possible with 18mm f/1.8 ?? I'm seriously thinking sell my J1 and buy a sony Nex F3 - it's the same price here in Brazil...

Yes, go with the bigger sensor either Sony, Canon or Samsung use DX sized sensors, not only will you get better control of DOF, better subject isolation, but you will also get significantly better ISO performance in low light and increased dynamic range, like someone already said, it's all physics, larger sensors are superior